Varec Biogas
A Division of Westech Industrial Inc.
5362 Oceanus Dr. Ste. A, Huntington Beach, CA 92649
p: 714.220.9920 | f: 714.952.2701
www.varec-biogas.com
VB-TN-002, Rev. A
August 16
R. HANSON
5000/5010 Series Flame Arresters –
Your Only Choice for Biogas Applications
In biogas systems, flame arresters are typically
installed on top of digester roofs or gas holders to
prevent any potential for igniting the biogas
accumulating in the digester. Flame Arresters are also
installed where there is an open flame or possible
sparking from compressors, flares, boilers or enginegenerators. Additional protection of utilizing thermal
shut-off valves and pressure (explosion) relief valves
are typically specified along with the flame arrester.
The Varec 5000/5010 Series Flame Arrester is a
square flame arrester (shown at right). The bank
assembly includes corrugated rectangular sheets
housed in an extensible frame (see photo below).
When neatly stacked together, the sheets form tiny
passages. These passages comprise the net free area
that will dissipate the heat and quench the flame.
How does it work?
The type of gas and the type of flame determines the size of
the passages.
Understanding flame characteristics
Upon ignition, a pressure shock wave occurs. The
compressed gases travel ahead of the flame. The flame
traveling through the pipe increases in velocity and pressure
(see chart-next page).
Overdriven detonation occurs at the point where the flame
ignites the compressed gases.
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The flame arrester should be able to sustain a flame flashback (deflagration), and in some
applications, the flame arrester must be able to stop the potential for detonation.
Gas Classification
Flame Arresters are also classified into four basic groups according to the reactivity of the
gas/vapor and air mixture application. Classification for biogas being primarily composed of
Methane is Group D in the United States, and Group 1 in Europe.
Gases are classified using the Maximum Safe
Experimental Gap (MESG). The MESG is the
maximum width of a gap, one inch (25mm) long
in an enclosure through which a flame cell of a
particular gas cannot pass through. The MESG
value is smaller for more reactive gases. The
tiny passages that are formed when the
corrugated rectangular sheets are neatly
stacked together on the flame arrester need to
be smaller than the MESG of the gas. This will
ensure that the flame cells are subjected to a
smaller area and assist in reducing its velocity
and temperature.
Arresters are tested and approved according to
the classification of the test gas selected by the
Flame Arrester Manufacturer. Flame Arresters tested to this gas can only be used for this type
of gas or for less reactive gases.
It is therefore important to note that the flame arrester specified should be particularly designed
to handle Group D – Methane Gas by accounting for the overall net free area. If a flame arrester
is misapplied, it will potentially compromise the device’s effectiveness and may not prevent
flame passage nor flame re-ignition within the arrester.
How is it better?
The 5000/5010 Flame Arrester has become the
standard for biogas application because its design
takes into account the nature of wet and dirty
biogas.
Net Free Area
A higher overall net free area creates the barrier
for flame front propagation. The position of the
rectangular sheets in the bank assembly allows
condensate or sediment to properly drain when
installed either in the vertical or horizontal position.
Proper draining prevents condensate pooling and
sediment build-up ensuring proper operation of the
flame arrester.
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Maintainability
Easy maintainability with the bank assembly prevents any need to remove the entire unit off
line. The connecting pipe flanges remain undisturbed when the cover plate is removed. Hence,
there is no stress applied to the connecting piping when maintaining the 5000/5010 flame
arrester.
Shorter maintenance downtime can be achieved when cleaning the bank assembly. First,
having a spare bank assembly handy for easy replacement while one is cleaned is important.
Next, the unit requiring cleaning can be removed from service by simply unbolting the fasteners
on the cover plate of the 5000/5010 Flame Arrester.
Cleaning is simplified as well. High spray wash water is not necessary for cleaning because the
low copper extensible frame allows the sheets to be individually inspected for any sediment
build-up. Simple soaking in solvents does the trick. You also eliminate the possibility of
distorting the flame element when using a high-pressure spray wash system for cleaning. Any
flame element distortion could represent a flame path. This is one of the main reasons why
Varec no longer sells individual sheets because possible mishandling of the sheets could lead
to a potential flame path. No margin of error is allowed!
A typical design common to flame arresters in the industry is shown on the left. Varec’s version
of the round configuration is the Model 5400 Series Flame Arrester. This is a product that Varec
strictly provides to the oil and gas industry where the media is traditionally cleaner and less
saturated.
Two pieces are shown on the photo (shown
left). The left is the entire unit, and the right
piece is the core assembly. The core
assembly is made of spiral-wound crimped
ribbon susceptible to clogging.
This core
assembly is what stops the flame from
propagating downstream of the ignition
source.
The round configuration or corrugated roll is
also susceptible to condensate pooling
because there is no place for the condensate
to properly drain especially when installed in a
horizontal position. When the flame arrester is
poorly maintained and clogged, then the
pressure loss through the unit is increased and
can potentially affect the entire plant control
system.
There are five distinct pieces in the round
flame arrester and they are as follows: (a) The
two end flange housings; (b) the two gaskets; and (c) the core assembly. In order to get to the
core assembly, the two end housings must be pried apart with the use of a jackscrew to extend
the housing. If a pressure and vacuum relief valve is sitting on top of this type of flame arrester,
then the pressure and vacuum relief valve must be properly supported or may require taking the
valve off line before removing the core assembly of the flame arrester. This is the same when
the arrester is installed in-line, it is important to ensure that the pipe is well supported prior to
removing the core assembly.
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Since it is difficult to tell if the crimped ribbon
design is clean and ready to be put back in
service, most operators use a high-pressure
spray wash system to clean the core
assembly. It is equally difficult to tell if there is
any distortion on the crimped ribbon after the
core has been spray-washed.
Putting together the five pieces correctly may
present a safety issue because if any of the
five pieces are misaligned when reassembled, then there is potential for gas
leakage, or worst, ignition.
How is it important?
We have heard the common response from
the industry that the potential for biogas
igniting in a low pressure and velocity piping system is so minute, that the need for a
sophisticated flame arrester is not a must. The photo on the next page is an actual case. Late 3
1996, a food-processing plant in Grand Forks, North Dakota installed a Varec Model 450 Flame
Trap Assembly upstream of a waste gas burner. The food-processing plant treats its own
wastewater estimated at ½ million gallons per day. The water is a by-product of the steam
utilized to power most of their food-processing equipment. The plant uses an anaerobic lagoon
that produces 300,000 scfd of biogas. Currently, all of the recovered biogas is run through the
flare, and completely burned. At the food processing plant, the flame arrester was installed off a
90° tee from the flare: before the thermal bypass shut-off valve. The standard installation is the
thermal shut-off valve should be installed immediately upstream of the potential flame source,
and in this instance, the thermal shut-off valve should have been installed immediately upstream
of the waste gas burner. According
to the Operations & Maintenance
(O&M) Manager of the wastewater
treatment sector, he checked the
Model 450 before he left for the
day. Upon his return eight hours
later, the flame already burned
through one of the flanges of the
flame arrester. A flashback from
the burner occurred sometime that
night.
In the O&M Manager’s
estimation, the flame arrester may
have actually sustained the flame
for more than four hours (picture
shown)! Despite the lapsed time, it
is important to note that arrester
held back the flame. Even the
thermal bypass shut-off valve
escaped unscathed.
Briefly put, the flame arrester did its job. An occurrence like this is something a plant hopes
never happens. In the event that it does, you have to ensure that the flame arrester you install
will do the job. Reliability is the key!
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